A cardinal mark is a sea mark (a buoy or other floating or fixed structure) commonly used in maritime pilotage to indicate the position of a hazard and the direction of safe water.

Cardinal marks indicate the direction of safety as a cardinal (compass) direction (north, east, south or west) relative to the mark. This makes them meaningful regardless of the direction or position of the approaching vessel, in contrast to the (perhaps better-known) lateral mark system.

The characteristics and meanings of cardinal marks are as defined by the International Association of Lighthouse Authorities.

A cardinal mark indicates one of the four compass directions by:

Either a quick or a very quick sequence of light flashes may be used; the choice allows for two similar nearby marks to be uniquely identified by their lights.

A cardinal mark may be used to accomplish the following:

Other uses:

The north and south topmarks are self-explanatory (both cones pointing up, or both pointing down). Remembering the east and west marks can be more of a problem.